Breast cancer frequently metastasizes to bone where it leads to osteolysis and poor clinical prognosis; however, the underlying roles of hydroxyapatite (HA) - a key component of breast microcalcifications (i.e., a negative prognostic factor for breast cancer) and the bone mineral matrix - remain unclear in this process, due in part to a lack of appropriate model systems. In the presence of a tumor, the physicochemical properties of HA (e.g., crystallinity, chemical composition, size, and aspect ratio) vary with disease state at both the primary (breast) and secondary (bone) sites. The overall hypothesis guiding the current investigator is: tumor-mediated changes to HA materials properties enhance breast cancer metastasis to bone by inducing a bone-metastatic phenotype at the primary site. These cells, in turn, promote premetastatic bone remodeling, which ultimately fosters bone colonization. We have previously developed mineral-containing 3-D tumor models, which permit testing of the importance of the physicochemical properties of HA in breast cancer spreading to bone. Using this system, coupled with advanced materials characterization techniques, we will test three subhypotheses: 1) HA in microcalcifications associated with more aggressive breast cancer is characterized by increased size and crystallinity and leads to the up-regulation of bone metastatic properties in breast cancer cells due in part to varied non-specific protein adsorption; 2) HA in the bones of tumor-bearing mice is characterized by decreased size and crystallinity even prior to metastatic colonization. These changes favor tumor cell seeding and growth, which are mediated by tumor-secreted endocrine signals that differentially regulate bone cell behavior; 3) Increased bone-metastatic potential of breast cancer cells due to interactions with HA enhances premetastatic bone remodeling, which, in turn, increases the osteotropism of breast cancer cells; pharmacological intervention with this process can decrease bone metastasis. There are three specific aims designed to test these hypotheses: In Aim 1, we will characterize HA materials properties in breast microcalcifications, and assess their impact on the bone-metastatic potential of tumor cells. In Aim 2, we will characterize HA materials properties in the bones of tumor-bearing animals pre- and post-colonization with breast cancer cells, and identify their role in secondary tumor formation. In Aim 3, we will assess the integrated effects of breast microcalcifications and premetastatic bone remodeling on breast cancer bone metastasis. The novel combination of cancer biology with engineering and materials science approaches will result in a highly reproducible and pathologically relevant culture platform that will allow us to deconvolute the complexity of bone metastasis and identify molecular targets for improved therapies. By elucidating the importance of materials-based mechanisms, the proposed research has the potential to challenge the currently accepted paradigm of bone metastasis as a disease that is solely mediated by cellular and molecular changes.